Speaking of Clutch fans, in particular on the 3B, here is what happened to me a few weeks ago.
I had just finished some minor repairs on my 86 BJ70 (AXT Trubo, lift with 33" tires, and stock 3.7 gears) I was driving home from my shop, and coming down a large grade (low EGT's low temps, etc) I stopped at a shop and and left the engine running, when I got back in a minute or so later I noticed the temp had jumped from 185f to 200f, I started driving and it jumped up to 2.05f, and then went back down to about 195f, thought maybe a fan clutch problem, but not that big of a deal (original fan clutch, my only 3b left with a fan clutch). Took the truck back out to shop the next weekend, and when I hit the large grade going up the temp consistantly went up, even when I nursed it up the hill with hardly any boost and low egts, it just kept going up, it settled at 210f and would not go much below that, so lets say for the next 20 minutes of driving no matter what I did it would fluctuate between 205 - 220f, once I got to shop I cooled down the engine and proceded to change the clutch fan with one of my solid clutch fans. after that all was well runs between 180 and 190 again. I guess I mention the story because the clutch fan can go, in an instant and when it does, there is more involved that just getting some airflow through the front of the Cruiser, it can act very strange, I felt pretty helpless.
What I notice that is probably very consistant with a 3B clutch fan going, is the temperature will steadily climb over time, and the engine can not recover very well from those temp jumps. I think the only thing that makes it worse, is if you get to a point when you blowing radiator fluid out fo the overflow, you problems just get worse if that happens.
I can tell you that with all the temps that I encountered with this, the head was safe and no cracks (I think this is one of th newer hi nickel heads).
Cheers,
Michael
